An audio compact disk ("CD") consists of a transparent polycarbonate substrate covered by a reflective material, which in turn is covered by a transparent protective layer. A spiral-shaped track is formed within the reflective material which contains successive shallow depressions, also called "pits," wherein digitally encoded information is stored. The length of the pits and the distance between the pits determine the value of the digitally encoded information.
A compact disk decoding system ("CD reader") is utilized to read the encoded information that is stored on a CD, and produce an audio and/or video output. The CD reader includes a read head and a digital signal processor ("DSP"). The read head directs a laser beam onto the CD track wherein the beam is diffracted by the pits in the reflective layer and then received by a photodetector. The photodetector measures the modulated optical signal that is diffracted back from the reflective layer and converts it into a corresponding digital signal for use by the DSP.
The DSP uses the modulated digital signal of the photodetector to produce corresponding decoded digital information. The decoded digital information comprises both digital audio information and digital "subchannel" information. The subchannel information is used for the purposes described below. The modulated signal of the photodetector contains 1,468,800 bits per second of digital information pertaining to the information encoded on the CD. This corresponds to 32 bits of digital audio information produced at 44.1 kilohertz, plus 96 bytes of digital "subchannel" information produced at 7.2 kilohertz (75 times per second). The DSP decodes and separates the digital audio information into two data words which respectively correspond to a left audio channel and a right audio channel. The left and right audio channel information is processed by digital-to-analog converters ("DAC's") that convert the 44.1 kHz digital audio information into analog signals for play on speakers. The DSP also decodes the 96 bytes of subchannel information and provides the decoded information to a central processing unit (CPU) within the CD reader in one of two ways, as discussed below.
Each byte of subchannel information contains 8 bits which are respectively labeled P, Q, R, S, T, U, V & W. The P bit, or P channel, is a flag bit that designates the start of an audio track (i.e, song 1, song 2, etc.). If the P channel equals O, audio information is being read from the CD. If the P channel equals 1, the start of a musical track is designated. In early CD readers, for example, when a user requested the playing of track 3, the processor within the CD reader would find the third occurrence of the P channel equal to 1 and would begin playing audio.
The Q bit, or Q channel, is used for position control in more sophisticated CD readers. The Q channel is encoded into 8-bit bytes to store position information such as track number, pause between music tracks, and time into each track in minutes, seconds and frames. The Q channel bits are read from the subchannel bytes, and de-serialized into 8-bit bytes for decoding by the CPU within the CD reader. Thus, for every eight subchannel bytes read, one 8-bit Q channel byte is available for decoding. Modern CD readers utilize the DSP to extract the Q channel information from the subchannel bytes, and decode the Q channel for position control.
The remaining R through W subchannel bits can be utilized for display of high resolution graphics during audio playback. Utilization of the R-W subchannels for display of graphics during audio playback is known as "CD+G". These six bits contain instructions for line or TV display of text or graphics, as well as scrolling or shifting of the text or graphics. The R-W subchannel bits can be extracted from the subchannel bytes to produce six bits of CD+G information for every subchannel byte that is read. The extracted R-W subchannel bits can then be presented to a host processing system for decoding and display during playback.
Most standard CD players are incapable of utilizing CD+G information to display graphics. When operating in a first mode, the DSP does not decode or provide the subchannel graphics bits R-W to the CPU within the CD reader and therefore does not enable display of the graphics information. Rather, the DSP decodes the P-W subchannel bytes, extracts and decodes the Q-channel bits, and provides the decoded Q-channel information to the CPU within the CD reader. The CPU then utilizes the decoded Q-channel information for the purpose of controlling the selection and play of audio. While the player operating in this mode is adequate to control and play audio, it is incapable of displaying text or graphics along with the audio because the subchannel graphics bits R-W are not decoded by the DSP.
The DSP within a standard CD reader may alternatively operate in a second mode that furnishes the R-W subchannel information to the CPU within the CD reader. In this mode, the subchannel information is provided to the CPU one byte at a time. However, when the DSP is operating in this second mode, the Q-channel bits are not extracted from of the subchannel, and are not de-serialized to provide timing information to the CPU within the CD reader. The CD reader therefore has no way to determine the position of the read head with respect to the CD.
Thus, standard CD readers can be placed in one of two modes. The first mode provides processed and de-serialized Q-channel timing directly to the CPU within the CD reader, but does not provide R-W subchannel information for display of graphics. The second mode provides R-W subchannel information as part of the P-W subchannel byte, but does not perform any processing or de-serializing of the Q-channel for purposes of timing.
An alternative that allows for processing of Q-channel information, as well as for providing subchannel bits R-W, requires the provision of an additional processor within the CD player to read the P-W subchannel information provided by the DSP, extract off the Q channel, and serialize the Q channel information for control and timing. Such a system allows for control and tracking of the audio information through processing of the Q-channel, and for display of graphics through provision of subchannels R-W. However, such an alternative is very costly due to the additional hardware and firmware required to process and decode the Q channel.
What is needed is an arrangement for a standard CD reader in which the P-W subchannel information from the DSP is processed, the Q-channel information is de-serialized, and the information in subchannels R-W is provided for display, utilizing the existing reader configuration and without requiring additional processor hardware.